For better abrasion resistance of cutting tools including a hard metal, cermet, or a high-speed tool steel as a substrate, the substrate surface has been coated with a hard coating such as a TiN, TiCN, or TiAlN coating. As hard coatings having better properties, for example, Patent Literature 1 discloses a coating that is based on TiN or CrN and further contains Si so as to have better oxidation resistance. Patent Literature 2 discloses a coating that is based on TiAlN and further contains Si so as to have better oxidation resistance. Patent Literature 3 discloses a coating that is based on TiCrAlN and further contains Si so as to have better oxidation resistance.
Patent Literature 4 discloses a coating that is based on CrAlN and further contains Nb, Si, and B so as to have better oxidation resistance Patent Literature 5 discloses a hard coating that comprises a metallic compound containing at least one metallic element M and at least one non-metallic element X. The metallic element M is selected from the group consisting of Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, and the non-metallic element X is selected from the group consisting of C, N, and O. Part of the metallic compound is substituted by at least one additional element L selected from the group consisting of Mn, Cu, Ni, Co, B, Si, S, Y, Ge, and Ga. Patent literature 5 fails to disclose the content of the added element L alone and fails to present an example using Ge, which is essentially used in the present invention.
As described above, various coatings have been proposed as hard coatings. However, with higher hardness of workpieces to be cut and higher cutting speeds, demands have been made to provide a mating having still better abrasion resistance.